This invention relates to an analog signal power amplifier circuit for amplifying an analog signal, especially a speech signal for driving a loudspeaker.
Speech synthesizer integrated circuits using digital signal processing techniques have been recently developed and are now commercially available. An integrated circuit for speech synthesis comprises CMOS transistors which entail a number of circuit elements. For this reason, in order to produce a large analog output current, the element dimensions at the output stage are greatly increased, thereby increasing chip size.
A speech synthesizer IC has been developed wherein an amplifier circuit thereof comprises a gain stage of MOS transistors and an output stage of a bipolar transistor. The bipolar transistor allows a large current to flow therethrough while being small in size. The circuit of this type is disclosed in Japanese Patent Publication (Kokai) No. 50-98756. However, since a CMOS inverter is used in the gain stage of this circuit, the operating point varies in accordance with a change in threshold of the CMOS inverter, thereby greatly varying the dynamic range of the output signal.